readme: fix header formatting

The first line was presumably supposed to be a markdown header,
but was missing a space after the `#`.  Also, renders a bit large to
as a level-1 header, so make it a level-2 header.

README.md

@@ -1,4 +1,4 @@
-#[explainshell.com](http://www.explainshell.com) - match command-line arguments to their help text
+## [explainshell.com](http://www.explainshell.com) - match command-line arguments to their help text
 
 explainshell is a tool (with a web interface) capable of parsing man pages, extracting options and
 explain a given command-line by matching each argument to the relevant help text in the man page.