docs: replace em-dash with colon in the JSON-library line

The em-dash rendered as an artifact on the Doxygen-derived pages. Use a plain
colon in the README and mainpage.dox "Required" lists.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Ben Collins <bcollins@libjwt.io>

README.md

diff --git a/README.md b/README.md
index c6b5baa..bc7e1cf 100644
--- a/README.md
+++ b/README.md
@@ -25,7 +25,7 @@
 
 ### Required
 
-- A JSON library — either [Jansson](https://github.com/akheron/jansson)
+- A JSON library: either [Jansson](https://github.com/akheron/jansson)
   (>= 2.0, the default) or [json-c](https://github.com/json-c/json-c)
   (>= 0.16, selected with ``-DWITH_JSON_C=ON``). The two are interchangeable.
 - [CMake](https://cmake.org) (>= 3.7)

doxygen/mainpage.dox

diff --git a/doxygen/mainpage.dox b/doxygen/mainpage.dox
index 502af7a..5820d1d 100644
--- a/doxygen/mainpage.dox
+++ b/doxygen/mainpage.dox
@@ -20,7 +20,7 @@
 
 @subsection req Required
 
-- A JSON library — either [Jansson](https://github.com/akheron/jansson)
+- A JSON library: either [Jansson](https://github.com/akheron/jansson)
   (>= 2.0, the default) or [json-c](https://github.com/json-c/json-c)
   (>= 0.16, selected with ``-DWITH_JSON_C=ON``). The two are interchangeable.
 - [CMake](https://cmake.org) (>= 3.7)