pkgs/stdenv: fix typo in booter.nix docs

pkgs/stdenv/booter.nix

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 # special snowflake due to different authors writing in different times.]
 #
 # The second goal is consistency across each stdenv's stage functions. By
-# writing each stage it terms of the previous stage, commonalities between them
+# writing each stage in terms of the previous stage, commonalities between them
 # are more easily observable. [Before, there usually was a big attribute set
 # with each stage, and stages would access the previous stage by name.]
 #