capitalize first letter in the error

Author: studavekar

mbed/mbed.py

@@ -1062,7 +1062,7 @@ def fromlib(cls, lib=None):
         if m_repo_ref:
             rev = m_repo_ref.group(3)
             if rev and not re.match(r'^([a-fA-F0-9]{6,40})$', rev):
-                error('named branches not allowed in .lib, offending lib is {} '.format(os.path.basename(lib)))
+                error('Named branches not allowed in .lib, offending lib is {} '.format(os.path.basename(lib)))
 
         if not (m_local or m_bld_ref or m_repo_ref):
             warning(