Fix typo: as => is

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets |

Author: ifdattic

contributing/code/conventions.rst

@@ -75,7 +75,7 @@ must be used instead (where ``XXX`` is the name of the related thing):
     While "setXXX" and "replaceXXX" are very similar, there is one notable
     difference: "setXXX" may replace, or add new elements to the relation.
     "replaceXXX", on the other hand, cannot add new elements. If an unrecognized
-    key as passed to "replaceXXX" it must throw an exception.
+    key is passed to "replaceXXX" it must throw an exception.
 
 .. _contributing-code-conventions-deprecations: