Update sec_gt-relations.ptx

Fix typos:

- "Ler" should be "Let"
- (a,a) is an element of the relation R, not the set A.

Author: suragch

source/sec_gt-relations.ptx

@@ -544,11 +544,11 @@
 
       <statement>
         <p>
-          Ler <m>R</m> be a relation on the set <m>A</m>.  We say,
+          Let <m>R</m> be a relation on the set <m>A</m>.  We say,
           <ul>
             <li>
               <p>
-                <m>R</m> is <term>reflexive</term> provided <m>(a,a) \in A</m> for all <m>a \in A</m>.
+                <m>R</m> is <term>reflexive</term> provided <m>(a,a) \in R</m> for all <m>a \in A</m>.
               </p>
             </li>
             <li>