[ci skip] fix typos

RELEASE.txt

@@ -37,9 +37,6 @@ CHANGED/ENHANCED EXISTING FUNCTIONALITY
 
 FIXES
 -----
-
-- List fixes of outright bugs
-
 - PackageVariable now does what the documentation always said it does
   if the variable is used on the command line with one of the enabling
   string as the value: the variable's default value is produced (previously
@@ -75,13 +72,13 @@ IMPROVEMENTS
   under which they would be observed), or major code cleanups
 
 - For consistency with the optparse "add_option" method, AddOption accepts
-  an SConsOption object as a single argument (this failed previouly).
+  an SConsOption object as a single argument (this failed previously).
   Calling AddOption with the full set of arguments (option names and
   attributes) to set up the option is still the recommended approach.
 
 - Add clang and clang++ to the default tool search orders for POSIX
   and Windows platforms. These will be searched for after gcc and g++,
-  respectively. Does not affect expliclity requested tool lists.  Note:
+  respectively. Does not affect explicitly requested tool lists.  Note:
   on Windows, SCons currently only has builtin support for clang, not
   for clang-cl, the version of the frontend that uses cl.exe-compatible
   command line switches.