Fix instructions for installting GDB on Fedora

Issue rust-embedded#364 reports that
the package arm-none-eabi-gdb has been dropped from Fedora (30-ish).
Their Bugzilla issue
https://bugzilla.redhat.com/show_bug.cgi?id=1859627#c23 recommends
installing the toolchain provided by arm and that is what the section
'Other distros' in the book does as well.

A gdb-multiarch package does not seem available at a first glance.

src/03-setup/linux.md

@@ -42,12 +42,10 @@ sudo apt-get install \
 
 ### Fedora 23 or newer
 
-> **NOTE** `arm-none-eabi-gdb` is the GDB command you'll use to debug your ARM
-> Cortex-M programs
+Install GDB as described in [Other distros](#other-distros) and the following packages:
 
 ``` console
 sudo dnf install \
-  arm-none-eabi-gdb \
   minicom \
   openocd
 ```