Update README.rst

typo

README.rst

@@ -21,7 +21,7 @@ There is a backwards incompatibility with the former Sage app distributed by
 sagemath.org which is slated for removal from Sage 9.3.  That app included a bash
 script at the top level of the bundle (e.g. /Applications/SageMath.app/sage).  But
 placing a script in that location prevents signing the bundle, since it violates
-Apple's specifications for the structure of a applicaton bundle.  Instead, this
+Apple's specifications for the structure of an application bundle.  Instead, this
 app provides a separate installer package which creates an executable bash script
 named /usr/local/bin/sage that can be used to run sage from a shell or script.