cmFindCommon: Fix typo in internal search path group label

Fix a typo from commit 1abd7cd930 (Use containers of labeled search
paths instead of individual members, 2014-10-17, v3.2.0-rc1~400^2).
The string is used only as an internal unique name, so the typo
was not user-facing.

Source/cmFindCommon.cxx

@@ -26,7 +26,7 @@ cmFindCommon::PathLabel cmFindCommon::PathLabel::CMakeEnvironment(
   "CMAKE_ENVIRONMENT");
 cmFindCommon::PathLabel cmFindCommon::PathLabel::Hints("HINTS");
 cmFindCommon::PathLabel cmFindCommon::PathLabel::SystemEnvironment(
-  "SYSTM_ENVIRONMENT");
+  "SYSTEM_ENVIRONMENT");
 cmFindCommon::PathLabel cmFindCommon::PathLabel::CMakeSystem("CMAKE_SYSTEM");
 cmFindCommon::PathLabel cmFindCommon::PathLabel::Guess("GUESS");