[libc] Fix typo in header generation docs.

[abrachet]

Generally speaking this kind of change can be pushed without review.

[abrachet]

Ping :)
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libc/docs/header_generation.rst

 Generating Public and Internal headers
 ======================================
 
 Other libc implementations make use of preprocessor macro tricks to make header
 files platform agnostic. When macros aren't suitable, they rely on build
 system tricks to pick the right set of files to compile and export. While these
 approaches have served them well, parts of their systems have become extremely
 complicated making it hard to modify, extend or maintain. To avoid these
 problems in llvm-libc, we use a header generation mechanism. The mechanism is
 driven by a *header configuration language*.
 
 Header Configuration Language
 -----------------------------
 
 Header configuration language consists of few special *commands*. The header
-generation mechanism takes a an input file, which has an extension of
+generation mechanism takes an input file, which has an extension of
 ``.h.def``, and produces a header file with ``.h`` extension. The header
 configuration language commands are listed in the input ``.h.def`` file. While
 reading a ``.h.def`` file, the header generation tool does two things:
 
 1. Copy the lines not containing commands as is into the output ``.h`` file.
 2. Replace the line on which a command occurs with some other text as directed
    by the command. The replacment text can span multiple lines.