Fix some documentation about access methods

Author: Guillaume Lelarge
Discussion: https://postgr.es/m/CAECtzeWPz4JikzUqZdMjqPTe8dAP3nZxPD-58Y-Hhvirg0fF+A@mail.gmail.com

Author: michaelpq

doc/src/sgml/catalogs.sgml

@@ -57,7 +57,7 @@
 
      <row>
       <entry><link linkend="catalog-pg-am"><structname>pg_am</structname></link></entry>
-      <entry>index access methods</entry>
+      <entry>relation access methods</entry>
      </row>
 
      <row>

doc/src/sgml/tablesample-method.sgml

@@ -264,7 +264,7 @@ NextSampleTuple (SampleScanState *node,
     requests to sample missing or invisible tuples; that should not result in
     any bias in the sample.  However, if necessary, the function can use
     <literal>node-&gt;donetuples</literal> to examine how many of the tuples
-    it returned were vlaid and visible.
+    it returned were valid and visible.
    </para>
   </note>