[spelling] existance to existence

tools/gn/docs/reference.md

 # GN Reference
 
 *This page is automatically generated from* `gn help --markdown all`.
 
 ## Contents
 
 *   [Commands](#commands)
     *   [analyze: Analyze which targets are affected by a list of files.](#analyze)
     *   [args: Display or configure arguments declared by the build.](#args)
     *   [check: Check header dependencies.](#check)
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   A list of files. Non-absolute paths will be resolved relative to the current
   build file.
 ```
 
 #### **Sources for binary targets**
 
 ```
   For binary targets (source sets, executables, and libraries), the known file
   types will be compiled with the associated tools. Unknown file types and
   headers will be skipped. However, you should still list all C/C+ header files
-  so GN knows about the existance of those files for the purposes of include
+  so GN knows about the existence of those files for the purposes of include
   checking.
 
   As a special case, a file ending in ".def" will be treated as a Windows
   module definition file. It will be appended to the link line with a
   preceeding "/DEF:" string. There must be at most one .def file in a target
   and they do not cross dependency boundaries (so specifying a .def file in a
   static library or source set will have no effect on the executable or shared
   library they're linked into).
 ```
 
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     *   [--root: Explicitly specify source root.](#--root)
     *   [--runtime-deps-list-file: Save runtime dependencies for targets in file.](#--runtime-deps-list-file)
     *   [--script-executable: Set the executable used to execute scripts.](#--script-executable)
     *   [--threads: Specify number of worker threads.](#--threads)
     *   [--time: Outputs a summary of how long everything took.](#--time)
     *   [--tracelog: Writes a Chrome-compatible trace log to the given file.](#--tracelog)
     *   [-v: Verbose logging.](#-v)
     *   [--version: Prints the GN version number and exits.](#--version)
 ```