match citation usage & syntax

[corybrunson]

The citations in the manuscript section "The Euler Characteristic Transform" are formatted parenthetically but treated as in-text; either the usage or the syntax should be fixed. For example, (See e.g. [@Hatcher]) should instead be (See e.g. @Hatcher) or perhaps just [@Hatcher]. (Here i'm referring to this section of the Pandoc manual.)

Part of this JOSS review.