fix typos

[spaette]

per request will attend to the following also

regarding Webinar by @jennybc

$ grep -nr classroms readxl
readxl/talks/2017-05-10_readxl-rstudio-webinar-notes.md:66:  * Bad news: Java. In my experience teaching classroms of ~80 with ~50/50 mix of Mac and Windows, ~30% of students can't get this working immediately on their own. The packages don't merely work poorly or slowly for these students; they simply do not work at all. I'm sure most of these problems can be solved with personalized expert help, but I can't offer that.
$ 

[jennybc]

This is vendored code, so I'm not going to take ?automated? suggestions to edit its comments. I feel like we have had a similar conversation before in another repo.

[spaette]

vendored code

my apologies

---

for my own records

the patch (to what appears to be abandonware) had furthermore been flawed

sed -i "s/normalizationis/normalization is/g" src/rapidxml/rapidxml.h
sed -i "s/occured/occurred/g" src/rapidxml/rapidxml.h
sed -i "s/precendence/precedence/g" src/rapidxml/rapidxml.h
sed -i "s/reponsibility/responsibility/g" src/rapidxml/rapidxml.h
sed -i "s/simplyfing/simplifying/g" src/rapidxml/rapidxml.h
sed -i "s/unneded/unneeded/g" src/rapidxml/rapidxml.h