Fix misspelling.

Signed-off-by: David A. Wheeler <[email protected]>

Author: david-a-wheeler

secure_software_development_fundamentals.md

@@ -37,7 +37,7 @@ This document is written to be easily understood by its audience (in this case,
 
 For more, see [5 Stupid Grammar Rules You Should Never Follow Because They Make Your Writing Worse](https://www.inc.com/minda-zetlin/5-stupid-grammar-rules-you-should-never-follow-because-they-make-your-writing-wo.html) and [6 Grammar Superstitions: The Never-Never Rules](https://wac.colostate.edu/docs/books/grammar/chapter6.pdf).
 
-Note that edX requires groupings at 3 levels: Sections, Subsections, and Units. Only Units (level 3) can have content.  We represent this in the material below as follows:  A “Heading 1” represents the beginning of a Section and contains ONLY sequences beginning with “Heading 2” (no text). A “Heading 2” represents the beginning of a Subsection and contains ONLY sequences beginning with “Heading 3”. A “Heading 3” represents the beginning of a Unit and contains all content. A Heading 3 cannot be directly contained within a Heading 1. Heading 4 (and below) are used freely within a Unit. Technically quizzes are level 3 in edX, but they are represented as level 4 in the markdown because the quizes are logically part of the section they quiz.
+Note that edX requires groupings at 3 levels: Sections, Subsections, and Units. Only Units (level 3) can have content.  We represent this in the material below as follows:  A “Heading 1” represents the beginning of a Section and contains ONLY sequences beginning with “Heading 2” (no text). A “Heading 2” represents the beginning of a Subsection and contains ONLY sequences beginning with “Heading 3”. A “Heading 3” represents the beginning of a Unit and contains all content. A Heading 3 cannot be directly contained within a Heading 1. Heading 4 (and below) are used freely within a Unit. Technically quizzes are level 3 in edX, but they are represented as level 4 in the markdown because the quizzes are logically part of the section they quiz.
 
 The primary editable content is in markdown format. There should be no markdownlint errors using our configuration. You can add hyperlinks to internal sections as #<i>section_name</i> where <i>section_name</i> is the title transformed using GitHub's [`gfm_auto_identifiers`](https://pandoc.org/MANUAL.html#extension-gfm_auto_identifiers) algorithm: space becomes `-`, uppercase (ASCII) becomes lowercase, and punctuation (other than `-` and `_`) are removed.