Fix doc for rustc "-g" flag

The rustc "-g" CLI flag was miss documented to be a synonym of "-C
debug-level=2" and not the correct "-C debuginfo=2".

Author: TheBiggerGuy

src/doc/rustc/src/command-line-arguments.md

@@ -42,7 +42,7 @@ This flag prints out various information about the compiler.
 
 ## `-g`: include debug information
 
-A synonym for `-C debug-level=2`.
+A synonym for `-C debuginfo=2`.
 
 ## `-O`: optimize your code