fix typo

Eric Sethna · web-flow · commit c825fba2385b · 2016-11-29T08:52:48.000-07:00
diff --git a/source/developer/fx-guidelines.rst b/source/developer/fx-guidelines.rst
@@ -418,7 +418,7 @@ Error message should not:
 - Say or have the term “Error”.
 - Be lengthy.
 - Be generic.
-- End in an exclaimation point.
+- End in an exclamation point.
 
 **Examples:**
 
