Update system-text-stringbuilder.md (dotnet#42299)

correct a spelling mistake, i.e, exact -> exert

Author: samibinsami

docs/fundamentals/runtime-libraries/system-text-stringbuilder.md

@@ -22,7 +22,7 @@ For example, the call to the <xref:System.String.Concat%2A?displayProperty=nameW
 :::code language="csharp" source="./snippets/System.Text/StringBuilder/Overview/csharp/immutability2.cs" id="Snippet1":::
 :::code language="fsharp" source="./snippets/System.Text/StringBuilder/Overview/fsharp/immutability2.fs" id="Snippet1":::
 
-For routines that perform extensive string manipulation (such as apps that modify a string numerous times in a loop), modifying a string repeatedly can exact a significant performance penalty. The alternative is to use <xref:System.Text.StringBuilder>, which is a mutable string class. Mutability means that once an instance of the class has been created, it can be modified by appending, removing, replacing, or inserting characters. A <xref:System.Text.StringBuilder> object maintains a buffer to accommodate expansions to the string. New data is appended to the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, and the new data is then appended to the new buffer.
+For routines that perform extensive string manipulation (such as apps that modify a string numerous times in a loop), modifying a string repeatedly can exert a significant performance penalty. The alternative is to use <xref:System.Text.StringBuilder>, which is a mutable string class. Mutability means that once an instance of the class has been created, it can be modified by appending, removing, replacing, or inserting characters. A <xref:System.Text.StringBuilder> object maintains a buffer to accommodate expansions to the string. New data is appended to the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, and the new data is then appended to the new buffer.
 
 > [!IMPORTANT]
 > Although the <xref:System.Text.StringBuilder> class generally offers better performance than the <xref:System.String> class, you should not automatically replace <xref:System.String> with <xref:System.Text.StringBuilder> whenever you want to manipulate strings. Performance depends on the size of the string, the amount of memory to be allocated for the new string, the system on which your code is executing, and the type of operation. You should be prepared to test your code to determine whether <xref:System.Text.StringBuilder> actually offers a significant performance improvement.