Correct outdated library name references

During the initial development of the library, it was named "POSIXStorage". The "Arduino_" prefix, which is used on the
names of all modern official libraries, was added later.

Some references to the library's name were not updated at the time of the renaming.

Author: per1234

extras/tests/Arduino_POSIXStorage_Test/Arduino_POSIXStorage_Test.ino

@@ -1,6 +1,6 @@
 /*
  *
- * POSIXStorage Tests
+ * Arduino_POSIXStorage Tests
  *
  * Original Author: A. Vidstrom (Arduino SA. http://arduino.cc)
  *

src/Arduino_POSIXStorage.cpp

@@ -69,7 +69,7 @@
   #include <Arduino_USBHostMbed5.h>
   #include <BlockDevice.h>
 #else
-  #error "The POSIXStorage library does not support this board"
+  #error "The Arduino_POSIXStorage library does not support this board"
 #endif
 
 /*