fix typo in databaseintro.html (#650)

db/databaseintro.html

@@ -42,7 +42,7 @@ Further conversion tools:
 
 <h3>SQL commands</h3>
 
-GRASS supports to main SQL operations, execution of an SQL statement 
+GRASS supports two main SQL operations, execution of an SQL statement 
 (<a href="db.execute.html">db.execute</a>) and selection
 of data from a table (<a href="db.select.html">db.select</a>).
 See the <a href="sql.html">SQL help page</a> for examples.