Use "file system" rather than "filesystem".

man/df.x

@@ -1,25 +1,25 @@
 [NAME]
-df \- report filesystem disk space usage
+df \- report file system disk space usage
 [DESCRIPTION]
 This manual page
 documents the GNU version of
 .BR df .
 .B df
-displays the amount of disk space available on the filesystem
+displays the amount of disk space available on the file system
 containing each file name argument.  If no file name is given, the
-space available on all currently mounted filesystems is shown.  Disk
+space available on all currently mounted file systems is shown.  Disk
 space is shown in 1K blocks by default, unless the environment
 variable POSIXLY_CORRECT is set, in which case 512-byte blocks are
 used.
 .PP
 If an argument is the absolute file name of a disk device node containing a
-mounted filesystem,
+mounted file system,
 .B df
-shows the space available on that filesystem rather than on the
-filesystem containing the device node (which is always the root
-filesystem).  This version of
+shows the space available on that file system rather than on the
+file system containing the device node (which is always the root
+file system).  This version of
 .B df
-cannot show the space available on unmounted filesystems, because on
+cannot show the space available on unmounted file systems, because on
 most kinds of systems doing so requires very nonportable intimate
-knowledge of filesystem structures.
+knowledge of file system structures.
 .SH OPTIONS

man/stat.x

@@ -1,4 +1,4 @@
 [NAME]
-stat \- display file or filesystem status
+stat \- display file or file system status
 [DESCRIPTION]
 .\" Add any additional description here

man/sync.x

@@ -1,4 +1,4 @@
 [NAME]
-sync \- flush filesystem buffers
+sync \- flush file system buffers
 [DESCRIPTION]
 .\" Add any additional description here