Update doc/pgtap.mmd

Co-authored-by: David E. Wheeler <[email protected]>

Author: dmfay

doc/pgtap.mmd

@@ -5091,7 +5091,7 @@ columns have a check constraint on them.
 `:description`
 : A short description of the test.
 
-Just like `col_is_check()`, except that it test that the column array has
+Just like `col_is_check()`, except that it tests that the column array has
 an exclusion constraint on them.
 
 ### `index_is_unique()` ###