binman: Fix SyntaxWarning: invalid escape sequence '\('

Reproduct warning:

    $ git clean -dfx
    $ make CROSS_COMPILE="" qemu-x86_64_defconfig
    $ make -j8
    ...
    u-boot/tools/binman/etype/section.py:25:
    SyntaxWarning: invalid escape sequence '\('
    """Entry that contains other entries

Signed-off-by: Rong Tao <rongtao@cestc.cn>

tools/binman/etype/section.py

@@ -40,7 +40,7 @@ class Entry_section(Entry):
     For example code, see etypes which subclass `Entry_section`, or `cbfs.py`
     for a more involved example::
 
-       $ grep -l \(Entry_section tools/binman/etype/*.py
+       $ grep -l \\(Entry_section tools/binman/etype/*.py
 
     ReadNode()
         Call `super().ReadNode()`, then read any special properties for the