fixed type in readme

README.md

@@ -68,14 +68,14 @@ Now in another terminal:
 ```
 cd <project_dir>/target/same54p20a/
 make
-{ESF_DIR}/toolchains/arm/bin/arm-none-eabi-gdb -iex "target extended-remote localhost:3333" testdir.elf
+${ESF_DIR}/toolchains/arm/bin/arm-none-eabi-gdb -iex "target extended-remote localhost:3333" testdir.elf
 load
 monitor reset
 ```
 
 If you're using gdb with python extensions then use this instead
 ```
-{ESF_DIR}/toolchains/arm/bin/arm-none-eabi-gdb-py -iex "target extended-remote localhost:3333" testdir.elf
+${ESF_DIR}/toolchains/arm/bin/arm-none-eabi-gdb-py -iex "target extended-remote localhost:3333" testdir.elf
 ```
 
 ### Supported Targets