The term “#define”, when the line breaking happens to put at the
beginning of a line, confuses markdown. This change escapes the '#'.

Change-Id: I8300324f9e8c7561f32aba6fa29c0132a188a58b

Change-Id: Ia825300bae236e3133dd9a19313b7f5450f0c8e2
Reviewed-on: https://boringssl-review.googlesource.com/7781
Reviewed-by: David Benjamin <davidben@google.com>